I think you did fine; but I agree with others that it will probably continue to happen. I think it's time to sit down with your supervisor and inquire as to how he/she wishes you to handle it in the future. It may well be that the supervisor will want to do so on her own and leave you out of it.

I have complained for a long time that my work email is only available from my physical work computer--but I'm becoming more and more convinced that this is probably for the best. If I got an email on a Saturday I might not be able to stop myself from answering it, then they would answer, then I would answer, and half my Saturday is gone. Instead when I read an email Monday morning that was sent Friday after 5pm, I know that I could not possibly have answered it any earlier, and I don't feel bad about it and can relax on the weekends. I just answer first thing Monday morning and don't even mention the delay in my email. The only time I mention it is if someone's weekend email literally says "can you send me such and such by 9pm tonight?" I feel that email still needs a response even though I obviously can't go back in time. So that's the only time I acknowledge the lack of immediate response in my return email. I just explain that I don't have access to email at home. I've never had someone question it further.

I agree about hourly vs. salary not necessarily being the issue here. Even people who are salaried have time when they are home and not working. Salary doesn't mean on-call all the time. Although Dh is salaried and has many responsibilities and sometimes has to work weekends when there is work that must be done; that still doesn't translate into the clients setting his work hours and deciding that he will be working on a particular evening or weekend. For an important client will he answer the phone on the weekend? Yes, if the voicemail indicates that it is actually urgent and can't wait until a regular workday. If it is something that can be done the next day? No. He is not going to be held hostage by a non emergent situation. If you don't apologize, it tends to send the message that you don't have anything to apologize for. I think Dh gets away with not returning those non urgent calls until Monday because he never says "oh sorry I didn't call back Saturday at 10pm". He just calls back Monday and immediately gets to business.